It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized.
An electromagnetic wave (and radio wave, specifically) has an electric field that varies as a sine wave within a plane coincident with the line of propagation, and the same is true for the magnetic field. The electric and magnetic planes are perpendicular and their intersection is in the line of propagation of the wave. If the electric-field plane does not rotate (about the line of propagation) then the polarization is linear. If, as a function of time, the electric field plane (and therefore the magnetic field plane) rotates, then the polarization is rotational. Rotational polarization is in general elliptical, and if the rotation rate is constant at one complete cycle every wavelength, then the polarization is circular. The polarization of a transmitted radio wave is determined in general by the transmitting antenna (and feed)—by the type of the antenna and its orientation. For example, the monopole antenna and the dipole antenna are two common examples of antennas with linear polarization. A helix antenna is a common example of an antenna with circular polarization, and another example is a crossed array of dipoles fed in quadrature. Linear polarization is usually further characterized as either vertical or horizontal. Circular Polarization is usually further classified as either Right Hand or Left Hand.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages for their optimization and utility. In general, antennas may be classified with respect to divergence or curl types, corresponding to dipoles and loops, and line and circle structures, as are well established.
Many structures are described as loop antennas, but standard accepted loop antennas are a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
A given antenna shape can be implemented in 3 complimentary forms: panel, slot and skeleton according to Babinet's Principle. For instance, a loop antenna may be a circular metal disc, a circular hole in a thin metal plate, or a circular loop of wire. Thus, a given antenna shape may be reused to fit installation requirements, such as into the metal skin of an aircraft or for free space. Although similar, the complimentary antenna forms may vary in driving impedance and radiation pattern properties, according to Booker's Relation and other rules.
Dual linear polarization (simultaneous vertical and horizontal polarization from the same antenna) has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system. Circular polarization in dipoles may be attributed to George Brown (G. H. Brown, “The Turnstile Antenna”, Electronics, 15, Apr. 1936). In the dipole turnstile antenna, two dipole antennas are configured in a turnstile X shape, and each dipole is fed in phase quadrature (0, 90 degrees) with respect to the other dipole. Circular polarization results in the broadside/plane normal direction. The dipole turnstile antenna is widely used, but a dual polarized loop antenna could be more desirable however, as full wave loops provide greater gain in smaller area. The gain of full wave loops and half wave dipoles are 3.6 dBi and 2.1 dBi respectively.
U.S. Published Patent Application No. 2008 0136720 entitled “Multiple Polarization Loop Antenna And Associated Methods” to Parsche et al. includes methods for circular polarization in single loop antennas made of wire. A full wave circumference loop is fed in phase quadrature (0°, 90°) using two driving points. Increased gain is provided relative to half wave dipole turnstiles, and in a smaller area.
Notch antennas may comprise notched metal structures and the notch may serve as a driving discontinuity for in situ or free space antennas. For example, notches can form antennas in metal aircraft skins, or they may electrically feed a Euclidian geometric shape. Euclidian geometries (lines, circles, cones, parabolas etc.) are advantaged for antennas. They are known for their optimizations: shortest distance between two points, greatest area for perimeter etc. Radiation properties of notch antennas may be hybrid between that of the driving notch and those of the notched structure.
U.S. Pat. No. 5,977,921 to Niccolai, et al. and entitled “Circular-polarized Two-way Antenna” is directed to an antenna for transmitting and receiving circularly polarized electromagnetic radiation which is configurable to either right-hand or left-hand circular polarization. The antenna has a conductive ground plane and a circular closed conductive loop spaced from the plane, i.e., no discontinuities exist in the circular loop structure. A signal transmission line is electrically coupled to the loop at a first point and a probe is electrically coupled to the loop at a spaced-apart second point. This antenna requires a ground plane and includes a parallel feed structure, such that the RF potentials are applied between the loop and the ground plane. The “loop” and the ground plane are actually dipole half elements to each other.
U.S. Pat. No. 5,838,283 to Nakano and entitled “Loop Antenna for Radiating Circularly Polarized Waves” is directed to a loop antenna for a circularly polarized wave. Driving power fed may be conveyed to a feeding point via an internal coaxial line and a feeder conductor passes through an I-shaped conductor to a C-type loop element disposed in spaced facing relation to a ground plane. By the action of a cutoff part formed on the C-type loop element, the C-type loop element radiates a circularly polarized wave. Dual circular polarization is not however provided.
However, there is still a need for a relatively small planar antenna for operation with any polarization including linear, circular, dual linear and dual circular polarizations.